The continuous shrinking of integrated circuits makes metrology much more difficult using ultraviolet-visible spectroscopy, because the wavelengths within this spectral range are much larger than the dimensions to be measured. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
With wavelengths that are more comparable to the structures to be measured, x-ray metrology is an attractive alternative. However, x-ray metrology techniques such as x-ray reflectometry, small angle x-ray scattering, x-ray fluorescence, x-ray diffraction and x-ray photo-electron spectroscopy impose significant challenges for x-ray sources if they are to meet the needs of the integrated circuit industry. Most prominent among those challenges are producing a sufficiently small beam spot with a sufficiently high beam brightness (also known as radiance, which is defined as the photon flux per unit solid angle per unit source area).
If the beam spot is not small enough, then a small feature or position within the integrated circuit cannot be measured without the measurement being affected by the surrounding area and hence potentially inaccurate. If the beam is not bright enough, then the signal to noise ratio will be too low for the required measurement accuracy and repeatability, and a long measurement time would be needed. The challenge of producing a sufficiently small spot of sufficiently high brightness is greatest when grazing incidence angles are required, such as in x-ray reflectometry and small-angle x-ray scattering, as well as some x-ray diffraction and x-ray fluorescence measurements.
Currently, only synchrotron radiation sources are able to provide an x-ray beam that meets these challenges. Unfortunately, synchrotron sources are large, expensive, and not well-suited for the fabrication environment where in-line metrology is performed on integrated circuits in a production environment.
What is needed, therefore, is an integrated circuit metrology system that overcomes problems such as those described above, at least in part.